This invention relates generally to the field of sonar and more particularly to improvements in high resolution, side-looking sonars for use in locating and classifying underwater objects.
Operational real or physical aperture side-looking sonars have sought to achieve high resolution by the use of high frequencies to effect a small angular beamwidth and correspondingly small lateral resolution at a long range. The lateral resolution .rho. is given as ##EQU1## where R is the target range, .lambda. is the wavelength, and L is the length of physical aperture. The acceptance angle .theta. for a sonar is given by the relationships ##EQU2##
Side-looking physical aperture sonars typically operate at frequencies and have required a lateral resolution L that dictate an acceptance angle .theta. of only about 1.degree. or less. Now, if a specularly reflecting target object, such as a man-made device having an elongate shape, has its major axis of reflection rotated so as lie outside the acceptance angle of the sonar, the likelihood of detection is materially reduced.
A sonar approach that has a large acceptance angle while still maintaining a lateral resolution comparable to current side-looking physical aperture sonars is that known as synthetic aperture sonar wherein a narrow side-looking beam is formed by transducer means on a vehicle moving along a path, and the returns received at a series of successive locations along that path are processed to provide signals representative of returns that would have been received by a long linear array of transducers at those locations. Thus, the effect of a large aperture and large acceptance angle are achieved. U.S. Pat. No. 4,088,978 to G. A. Gilmour is exemplary of such a sonar. In addition to signal processing to achieve the synthetic aperature effect due to linear forward motion of the transducer, additional complexities and limitations arise, including but not limited to those resulting from non-uniform, non-linear excursions of the vehicle from an ideal linear path of travel which degrades the detection capabilities below the theoretically high levels that would otherwise be attainable.
In some side-looking sonars of the fixed aperture variety, for example for bottom mapping purposes, the insonifying beam as well as the receiving beam is narrow in the direction of travel. When the supporting vehicle has a substantial forward speed, it has been found advantageous to skew the insonifying beam somewhat forwardly of the receiving beam so that in the time it takes the insonifying pulse to reach the target strip, the movement of the vehicle will bring the receiving beam to bear thereon. The application of such skew angles between transmitting and receiving beams is discussed in U.S. Pat. No. 3,585,578 to Raymond C. Fisher, Jr.
Other efforts to improve the likelihood of detection of small or poorly reflecting targets in side-looking sonars have included the use of electronic phasing of return signals outputs from segmented transducers so as to focus at particular ranges of interest as described in U.S. Pat. No. 3,950,723 to G. A. Gilmour, and the use of acoustic lenses to gather reflected energy and focus it on receiver transducers as exemplified in U.S. Pat. No. 3,895,340 to the same party.
Another prior art sonar technique for increasing object detection and classification involves the processing of return signals to detect the presence of acoustic shadows. An example of such in the side-looking sonar art is found in U.S. Pat. No. 4,030,096 to W. E. Stevens et al. It is considered desirable that improvements in side-looking sonar systems preserve or enhance the capability of using acoustic shadowing in target detection and classification.